It can be up to a 10-marker, so you need to prep enough for that.
Regarding marks-per-point, it's not advisable to plan to receive one mark per point you make: you might make a crappy point, word it crappily, link it poorly to the question, or just say something the assessor doesn't think sounds valid. Also, it's ignoring the fact that larger questions are marked globally - on overall quality, and not just number of points. A better way is to judge it on time writing, and combine that with an approximate length to make something detailed but not laboured.
Therefore:
You need to write for 20-25mins for the final 10-marker. It takes a little longer per mark to make it really good. You also need to make sure you get on to the third page at least. None of this is official 'assessment guideline', mind you - this is just playing it safer. It takes about three lines to make a good point - sometimes a little shorter, sometimes a little longer, but not varying wildly. If you put these things together, how many arguments do you need to fill 2-4 pages with around three lines per point and taking around 20-25mins to write? That's how many points you need.
Also remember that you can match more than one strength to a single weakness, and vice versa.